Wnts/wg are a family of conserved signaling molecules that have been shown to regulate a plethora of fundamental developmental and cell biological processes, including cell proliferation, differentiation and cell polarity [Miller et al. Oncogene 18, 7860-72 (1999); Polakis. Genes Dev 14, 1837-51 (2000); Wodarz et al. Annu Rev Cell Dev Biol 14, 59-88 (1998)]. Mutations in the Wnt genes or in those genes encoding regulators of the Wnt/wg signaling pathway can cause devastating birth defects, including debilitating abnormalities of the central nervous system, axial skeleton, limbs, and occasionally other organs [Ciruna et al. Nature 439, 220-4 (2006); Grove et al. Development 125, 2315-25 (1998); Jiang et al. Dev Dyn 235, 1152-66 (2006); Kokubu et al. Development 131, 5469-80 (2004); Miyoshi et al. Breast Cancer Res 5, 63-8 (2003); Shu et al. Development 129, 4831-42 (2002); Staal et al. Hematol J 1, 3-6 (2000)]. Aberrant Wnt signaling has also been linked to human disease, such as hepatic, colorectal, breast and skin cancers [Miyoshi et al. supra (2003); Miyoshi et al. Oncogene 21, 5548-56 (2002); Moon et al. Nat Rev Genet 5, 691-701 (2004)].
Wnts/wg encode secreted glycoproteins that activate receptor-mediated pathways leading to numerous transcriptional and cellular responses [Wodarz et al. supra (1998); Moon et al. supra (2004); Nusse. Trends Genet 15, 1-3 (1999)]. The main function of the canonical Wnt pathway is to stabilize the cytoplasmic pool of a key mediator, β-catenin (β-cat)/armadillo (arm), which is otherwise degraded by the proteosome pathway (See FIG. 1). Initially identified as a key player in stabilizing cell-cell adherens junctions, β-cat/arm is also known to act as a transcription factor by forming a complex with the LEF/TCF (Lymphoid Enhancer Factor/T Cell Factor) family of HMG-box (High mobility group) transcription factors. Upon Wnt stimulation, stabilized β-cat/arm translocates to the nucleus, wherein together with LEF/TCF transcription factors, it activates downstream target genes [Miller et al. supra (1999); Staal et al. supra (2000); Nusse. supra (1999); Schweizer et al. Proc Natl Acad Sci USA 100, 5846-51 (2003)]. Catenin responsive transcription (CRT), which is the activation of transcriptional targets of β-cat, has been shown to regulate many aspects of cell growth, proliferation, differentiation and death. The Wnt/wg pathway can also be activated by inhibiting negative regulators such as GSK-3β (Glycogen Synthase Kinase-3β), APC (Adenomatous Polyposis Coli) and Axin that promote β-cat/arm degradation, or by introducing activating mutations in β-cat that render it incapable of interacting with the degradation complex, thus stabilizing its cytosolic pool [Logan et al. Annu Rev Cell Dev Biol 20, 781-810 (2004); Nusse et al. Cell Res 15, 28-32 (2005)]. Wnt/wg signaling can also activate an alternative “non-canonical” pathway that may lead to PKC (Protein Kinase C) and JNK (c-Jun N-terminal Kinase) activation resulting in calcium release and cytoskeletal rearrangements [Miller et al. supra (1999)].
At the plasma membrane, Wnt proteins bind to their receptor, belonging to the Frizzled family of proteins and the co-receptor encoded by LDL-related-protein-5, 6 (LRP5, LRP6)/arrow (arr, in Drosophila) [Schweizer et al. BMC Cell Biol 4, 4 (2003); Tamai et al. Mol Cell 13, 149-56 (2004)]. In the absence of the Wnt stimulus, GSK-3β is known to phosphorylate β-cat/arm, which marks it for ubiquitination and subsequent proteosome-mediated degradation. Activation of the receptor/co-receptor complex upon Wnt binding initiates a signal transduction cascade, which results in phosphorylation and subsequent inactivation of GSK-3β24.
Recent evidence has uncovered a new branch in the canonical Wnt/wg pathway whereby β-cat/arm can be stabilized in a GSK-3β independent fashion suggesting that regulated degradation of β-cat/arm (by GSK-3β) is not necessary for Wnt/wg signaling [Tolwinski et al. Dev Cell 4, 407-18 (2003); Tolwinski et al. Trends Genet 20, 177-81 (2004)]. Specifically, upon Wg binding, Arr directly recruits Axin (a scaffold protein which acts as a negative regulator) to the plasma membrane and causes its degradation. As a consequence, Arm no longer binds Axin or the degradation complex, resulting in nuclear accumulation and signaling by β-cat/Arm42.
In view of the above, a need exists for therapeutic agents, and corresponding pharmaceutical compositions and related methods of treatment that address conditions causally related to aberrant Wnt pathway activity and CRT activity, and it is toward the fulfillment and satisfaction of that need, that the present invention is directed.